Long persistent luminescent materials are luminescent materials which store energy during the application of excitation light and emit light using the stored energy even after the application of the excitation light stops. Long persistent luminescent materials are used as luminescent paints for watch and clock dials which glow in the dark or at night, letters and diagrams of signs and guideboards and the like, and these materials have been recently applied to lights which can produce light with stored light energy without electricity supply.
Of these long persistent luminescent materials, inorganic salts containing rare-earth elements such as Eu, Ce and Tb are especially known as long persistent luminescent materials which exhibit long-lived luminescence (for example, see PTL 1).